The present invention relates to a friction clutch or friction brake having lubricated friction surfaces and components thereof. The invention further relates to friction clutches or brakes that are non-planar, preferably conical. In particular, the invention relates to a synchronizing ring and/or correcting ring for motor vehicle gear-shift mechanisms, consisting of a basic body made of a basic material and a friction body applied or attached to said basic body at least to partial zones thereof in a form-locking, friction-gripping and/or material-locking way.
The extensive field of friction clutches and friction brakes can be divided into different types, based on the various designs, type of friction pairings, lubrication, and the way the mechanism of switching forces works. With respect to the design of the friction surfaces, a distinction can be made between jaw clutches, cone clutches and disk clutches. Depending on the type of friction pairing and lubrication, a distinction is also made between dry-running and lubricated friction surfaces, with and without a special friction facing. Particularly with friction facings which are oil lubricated, the type of handling or control and the switching force are of great importance. During the switching action, the oil has to be forced out from between the friction surfaces in order to achieve friction grip with high coefficients of friction. If pneumatic or hydraulic servomotors are available for this purpose, high switching forces can be readily produced, which expel the oil from between the pairs of friction surfaces. This is different with manually shifted friction clutches or friction brakes such as, for example, manually shifted synchronized gears in motor vehicles, where one is faced with the problem that the friction surfaces float on each other so that oil may be expelled from between the friction surfaces too slowly, with the result that the switching time is undesirably long or, in the extreme case, that the synchronizing system fails to function properly.
The switching or braking properties of friction clutches or friction brakes are determined mainly by the friction characteristics of the friction pairings and the size of the actually acting friction surface. The actually acting friction surface is the portion of the geometric friction surface which, when engaged, is in actual friction grip with the partner surface. Friction surfaces have a more or less structured configuration, which reduces the actually effective or acting friction surface. Lubricated friction surfaces have draining grooves, channels or furrows in the friction surface for removing the oil when the friction grip is produced between the surfaces of the paired friction facings. Such grooves where indispensable until now, at least with manually shifted friction clutches and friction brakes, and required a reduction of the actually acting friction surface by 30 to 80% of the geometric friction surface.
Particularly with respect to non-planar paired friction surfaces, the shape of the one friction partner may be different from the shape of the surface of the other partner due to tolerances in the manufacture, or due to deformation under frictional pressure, so that the actually acting friction surface is often substantially reduced as compared to the geometric friction surface.
Friction surfaces are subjected to high shearing stresses and thermal loads, and to frictional wear that increases with temperature. To keep such wear low, the actually acting and, consequently, the geometric frictional surfaces should be as large as possible. The high requirements with respect to resistance to wear make it necessary to manufacture the friction facing from high-quality, expensive materials. However, for reasons of material savings, and thus cost savings, the frictional surfaces are designed with sizes as small as possible. Highly stressed modern friction couplings and friction brakes are mainly produced with shaped parts or components made of basic materials such as steel, brass and various bronzes, to which specific friction facings are applied by a variety of methods.
German patent DE 31 22 522 A1, for example, describes a synchronizing device for form-locking gear-shift clutches in which the synchronizing ring and/or correcting ring is made of metal and the friction facing is made of a ceramic material, whereby the ceramic material is both the supporting element and the material of the friction surface which is combined with the synchronizing ring and/or correcting ring by form-locking, friction-locking or material-locking. Alternatively, the ceramic material is applied to the latter in pulverized powder form by sputtering. The friction facing is manufactured according to this device from oxide and/or silicide and/or boride and/or carbide and/or nitride and/or titanium oxide.
However, a number of the problems discussed above occur if synchronizing rings and correcting rings according to the above patent are not modified. The materials used for the friction facing according to DE 31 22 522 are particularly expensive mainly if said facings are designed as self-supporting shaped bodies. Moreover, draining grooves cannot be dispensed in this apparatus, as a result the actually acting friction surface is reduced in size and the geometric friction surface must therefore be larger and thus more costly. The friction facing of the German patent is applied by sputtering, which is also costly. Moreover, as a rule, only one surface of the paired friction surfaces is coated with said highly abrasively acting friction body, the uncoated counter surface of the German patent is subjected to above-average wear. Furthermore, said design poses special problems with respect to heating of the coating, dissipation of the heat and adhesion of the friction facing.
In another known design for a synchronizing ring according to German patent DE 27 44 994, said ring is made of a metallic basic body and a glued friction body thereon. The friction body is paper based and has a number of organic friction materials impregnated therein with synthetic resin.
Such friction facings can be produced at relatively favorable costs; however, only low, frictional compression is permitted due to the low strength of such surfaces. Consequently, relatively large geometric friction surfaces are needed for the synchronizing rings, which means that such rings have to be large and, consequently, expensive. Large dimensions, however, are often not possible because of available design space limitations. Also according to this design, when the grip is produced, the film of oil can only be removed slowly from the large friction surfaces, resulting in a long duration of the synchronizing action. Furthermore, the friction characteristics are changed because the lubricated oil is decomposed at the high temperature experienced using this design. Consequently carbon from the decomposed oil is collected and embedded in the surfaces of the design, due to the cracking of the oil at high friction temperatures, leading to failure of the synchronizing device.
German utility patent No. 73 42 680 describes a synchronizing ring made of sintered steel, to which there is applied a thin coating of molybdenum within the zone of the friction surface. Molybdenum powdered material is expensive. As a rule, it has to be compacted further after it has been applied to the ring. In another manufacturing step, the device has to be provided with draining grooves, which means that such synchronizing rings are very costly. The coating so applied has a relatively high degree of roughness, which means that the actually acting friction surface is comparatively small. In practical applications, this means that the friction surface is excessively heated locally, resulting in corrosion of the synchronizing ring and correcting ring and detachment of the molybdenum coating and, in turn, substantial functional problems with the clutch.
The present invention, therefore, is directed to the problem of designing and producing friction clutches or friction brakes of the type specified above in such a way that their manufacture is possible at low cost while permitting a high degree of exploitation of the geometric friction surface, a long service life, high frictional overloads for brief periods and only minor premature failures with the above-mentioned drawbacks of the known designs substantially reduced or eliminated.